Classification
Therearebasicallytwotypesofmultimodefibers,oneisgradedandtheotherisstepped.Forgradedfibers,Inotherwords,therefractiveindexofthecoreisthesmallestattheperipheryofthecoreandgraduallyincreasestowardthecenterpoint,therebyreducingthemodaldispersionofthesignal.Forsteppedindexopticalcables,therefractiveindexisbasicallynotaverage.Change,andonlyonthesurfaceofthecladdingwillitsuddenlydecrease.Steppedfibersgenerallyhavealowerbandwidththangradedfibers.Innetworkapplications,themostpopularmultimodefiberis62.5/125,62.5/125meansthatthefibercorediameteris62.5μmandthecladdingdiameteris125μm,andtheothermorecommononesare50/125and100/140.
Comparison
Comparedwithtwistedpair,multimodefibercansupportlongertransmissiondistance.In10mbpsand100mbpsEthernet,multimodefibercansupportupto2000metersInthe1GbpSgigabitnetwork,themultimodefibercansupportatransmissiondistanceof550meters(50μmcorediameter).Ina10Gbps10Gbpsnetwork,themultimodefiberOM3canreach300metersandtheOM4canreach500meters.
SelectionGuide
Thenominaldiameterofthecorewireofthemultimodefiberis62.5μm/125μm.or50μm/125μm.Specifications(numberofcores)are2,4,6,8,12,16,20,24,36,48,60,72,84,96cores,etc.Theoutersheathmaterialofthecablehasordinarytype;ordinaryflameretardant;low-smokehalogen-freetype;low-smokehalogen-freeflame-retardanttype.
Applicationpotential
Marketinthe1990s
Inthe1990s,multimodeopticalfiberhasalwayshadastableshareintheworldopticalfibermarket.Forehead.Sincethemid-1990s,theworldmultimodeopticalfibermarkethasbasicallymaintaineda7-8%fiberconsumptionanda14-15%salesshare.NorthAmericahasahigherratiothanthisapproximateaverage.
Aftertheriseinthe1970s
Inthe1970s,opticalfiberenteredthepracticalstagefromtheinterofficerelayofmultimodeopticalfiber.Overthepasttwodecades,newvarietiesofsingle-modeopticalfibershavecontinuedtoappear.Thefunctionsofopticalfibershavebeencontinuouslyenrichedandenhanced,andtheperformance-priceratiohascontinuedtobedemanding.However,multi-modeopticalfibershavenotbeenreplacedbuthavealwaysmaintainedastablemarketshareanddevelopedsimultaneouslywithothervarieties.Thereasonisthatthecharacteristicsofmultimodefiberjustmeettherequirementsofnetworkfiber.Comparedwithlong-distancetrunklines,thecharacteristicsofopticalfibernetworksare:relativelylowtransmissionrate;relativelyshorttransmissiondistance;morenodes,morejoints,andmorebends;largeamountofconnectorsandcouplers;smallscale,andthenumberoflightsourcesusedperfiberlengthmany.
Features
Thelowtransmissionrateandshorttransmissiondistancecanjusttakeadvantageofthecharacteristicsofmultimodefiberbandwidthandtransmissionlossnotasgoodassinglemodefiber.Butsingle-modefiberischeaperandhasbetterperformancethanmulti-modefiber.Whyissingle-modefibernotusedinthenetwork?Thisisbecauseintheabove-mentionednetworkcharacteristics,thelossofmultiplebendsislarge;thenumberofnodesmeansthattheopticalpowersplitsfrequently,whichrequiressufficientopticalpowertransmissioninsidetheopticalfiber.Multi-modefiberhasalargercorediameterandlargernumericalaperturethansingle-modefiber,andcancouplemoreopticalpowerfromthelightsource.Theamountofconnectorsandcouplersinthenetworkislarge.Single-modeopticalfiberpassivecomponentsaremoreexpensivethanmulti-modeopticalfibers,andarerelativelypreciseandhavesmalltolerances.Theoperationisnotasconvenientandreliableasmulti-modecomponents.Single-modefibercanonlyuselaser(LD)asalightsource,anditscostismuchhigherthanthatoflight-emittingdiode(LED)usedinmulti-modefiber.Inparticular,thenetworkscaleissmall,andthenumberoflightsourcesusedperunitfiberlengthislarge.Onelightsourcemaybeusedforseveralhundredkilometersinthetrunkline,andeachnetworkofmorethantenkilometersorevenafewkilometershasitsownindependentlightsource.Ifthenetworkusessingle-modefiberwithlasers,theoverallnetworkcostwillbegreatlyincreased.Theverticalcavitysurfaceemittinglaser(VCSEL)hasbeencommerciallyavailable,anditspriceisclosetothatofLEDs.ItscircularbeamsectionandhighmodulationratejustcompensatefortheshortcomingsofLEDs,andmaketheapplicationofmultimodefiberinthenetworkmorevigorous.Itisnotdifficulttoseefromtheaboveanalysisthatitisnotcomprehensivetoconsiderthatsingle-modefiberhashighbandwidthandlowloss,andcanbeusedinthenetwork"onceinplace".Corninghascalculatedandcomparedthesystemcostofusingsingle-modefiberandmulti-modefiberinthenetwork.Thecostofusingsingle-modefiberisfourtimesthatofmulti-modefiber.Thesystemcostofusing62.5μmand50μmmultimodefiberisthesame,thedifferenceliesinthedifferenttypesofconnectors.Thecostofthenon-metalferrulepluggableconnectorsystem(multi-modesystemB)canbereducedby1/2comparedtotheconnectorwithametalferrulescrewconnection,suchasFCtype(multi-modesystemA).
Rise
Inordertomeettheneedsofnetworkcommunication,fromthelate1970stotheearly1980s,countriesvigorouslydevelopedlargecorediameterlargenumericalaperturemulti-modeOpticalfiber(alsoknownasdataopticalfiber).Atthattime,theInternationalElectrotechnicalCommissionrecommendedfourgradedindexmultimodefiberswithdifferentcore/cladsizes,namelyA1a,A1b,A1candA1d.Theircore/claddingdiameter(μm)/numericalapertureare50/125/0.200,62.5/125/0.275,85/125/0.275and100/140/0.316,respectively.Generallyspeaking,alargecore/packagesizeresultsinhighproductioncosts,poorbendingresistance,andanincreaseinthenumberoftransmissionmodulesandadecreaseinbandwidth.Inadditiontotheaboveshortcomingsof100/140μmmultimodefiber,itscladdingdiameteristoolarge,whichdoesnotmatchthetestequipmentandconnectingdevices,andwillnotbeusedindatatransmissionsoon,butisonlyusedforspecialoccasionssuchaspowertransmission.The85/125μmmultimodefiberhasbeengraduallyeliminatedduetosimilarreasons.TheIEC"SC"86A"GW1expertgroupmeetingheldinKyoto,JapaninOctober1999revisedthemultimodeopticalfiberstandard.InthereviseddraftpublishedinMarch2000,the85/125μmmultimodeopticalfiberhasbeencancelled.The50/125μmmultimodefiberdevelopedbyCorningin1976andthe62.5/125μmmultimodefiberdevelopedbyLucentBellLabsin1983havethesameouterdiameterandmechanicalstrength,buthavedifferenttransmissioncharacteristics.Theyhavealwaysbeen"competitive"indatacommunicationnetworks.".
Advantages
62.5μmcorediametermultimodefiberhaslargercorediameterandhighernumericalaperturethan50μmcorediametermultimodefiber,whichcanbecoupledfromLEDlightsourceMoreopticalpower,sothe62.5/125μmmultimodefiberwasfirstadoptedbytheUnitedStatesasanumberofindustrystandards.SuchasAT&T'sindoorwiringsystemstandard,AmericanElectronicsIndustryAssociation(EIA)LANstandard,AmericanNationalStandardsInstitute(ANSI)100Mb/stokennetworkstandard,IBM'scomputeropticalfiberdatacommunicationstandard,etc.50/125μmmultimodefiberismainlyusedasadatacommunicationstandardinJapanandGermany,andithasahistoryof18years.However,duetothelargeamountofopticalfiberinNorthAmericaandtheleadingroleofopticalfibermanufacturingandapplicationtechnologyintheUnitedStates,mostcountries,includingmycountry,use62.5/125μmmultimodeopticalfiberasalocalareanetworktransmissionmediumandindoorwiring.Sincethemid-1980s,62.5/125μmopticalfiberhasalmostbecomethemainstreamproductinthedatacommunicationopticalfibermarket.
Follow-updevelopment
Theabovesituationwasmaintaineduntilthemidtolate1990s.Inrecentyears,withthecontinuousupgradeofthetransmissionrateofthelocalareanetwork,the50μmcorediametermultimodefiberhasattractedmoreandmoreattention.Since1997,thelocalareanetworkhasdevelopedto1Gb/s,andthebandwidthofseveralhundredmegabytesof62.5/125μmmultimodefiberwithLEDasthelightsourceobviouslycannotmeettherequirements.Comparedwith62.5/125μm,50/125μmfiberhassmallernumericalapertureandcorediameter,anditsbandwidthishigherthanthatof62.5/125μmfiber,andtheproductioncostcanbereducedby1/3.Therefore,industriesinvariouscountrieshaveproposedtore-enable50/125μmmultimodefiber.Afterresearchanddemonstration,theInternationalOrganizationforStandardizationhasformulatedcorrespondingstandards.However,consideringthataconsiderablenumberof62.5/125μmmultimodefibershavebeeninstalledandusedinlocalareanetworksinthepast,theIEEE802.3zGigabitEthernetstandardstipulatesthat50/125μmand62.5/125μmmultimodefiberscanbeusedas1GMbit/sEthernet.Thetransmissionmediumused.Butfornewnetworks,50/125μmmultimodefiberisgenerallypreferred.There-launchof50/125μmmultimodefiberhaschangedthesituationwhere62.5/125μmmultimodefiberdominatesthemultimodefibermarket.Followingthesestandards,CorningannouncedthelaunchoftwonewmultimodefibersinSeptember1998.ThefirsttypeisInfiniCor300,accordingtothe62.5/125μmstandard,itcantransmit300metersatawavelengthof850nmand550metersatawavelengthof1300nmatarateof1Gb/s.ThesecondistheInfiniCor600type.Accordingtothe50/125μmstandard,atarateof1Gb/s,both850nmand1300nmwavelengthscantransmit600meters.
Impactoftherelease
AlthoughthenewlyreleasedIEEE802.3zstandardin1998proposedaspecificationfortheuseofmultimodefiberin1Gbit/snetworks,Thedevelopmentofnetworkupgradesisfasterthantheformulationofstandards.Thismakesthebandwidthlimitationof62.5/125μmmultimodefibermoreprominent.Inordertosolvethisproblem,suchasCorning'sInfiniCorCL1000andInfiniCorCL2000,Lucent'sLazr-SPEED,Alcatel'sGIGAliteandsoon.Corningsaidinthereleaseofthisopticalfiber:"Corningusesskilledtechnologyandnewrefractiveindexdistributioncontroltointroducethistypeofcharacteristicsthatonlysingle-modeopticalfibercanprovidebeforeandcanbeusedinthenetwork.Costsystem."
Thenewgenerationofmultimodeopticalfiber
Onthebasisoftheabovebackground,majorcompaniessuchasCorningandLucentoftheUnitedStateshaveproposed"Thenewgenerationofmultimodeopticalfiber"concept.ThestandardofthenewgenerationofmultimodeopticalfiberisbeingresearchedanddraftedbytheInternationalOrganizationforStandardization/InternationalElectrotechnicalCommission(ISO/IEC)andtheUnitedStatesTelecommunicationsIndustryAssociation(TIA-TR42).ItisexpectedtobelaunchedfromMarchtoApril2002.Anewgenerationofmultimodeopticalfiberwillalsobeusedasthetransmissionmediumof10Gb/sEthernetandbeincludedintheIEEE10Git/sEthernetstandard.TheEnglishabbreviation"NGMMF"(NewGenerationMultiModeFiber)forthenewgenerationofmultimodefiberhasbeenusedinternationallyandcanbeusedasakeywordtosearchoninternationalwebsites.Thecomprehensivetechnicalindicatorsofthenewgenerationofmultimodeopticalfiberhavenotbeenofficiallyannounced,butthefollowingexactinformationcanbeobtainedfromtherelevantreportsofthestandardformulationandrelatedtechnicalwebsites:
Type
ThenewgenerationofmultimodeThefiberisa50/125μmmultimodefiberwithagradedrefractiveindexprofile.The50μmcorediameterisusedbecausethenumberoftransmissionmodesinthisfiberisapproximately1/2.5ofthatina62.5μmmultimodefiber.Thiscaneffectivelyreducethemodaldispersionofthemultimodefiberandincreasethebandwidth.For850nmwavelength,thebandwidthof50/125μmcomparedto62.5/125μmmultimodefibercanbeincreasedthreetimes.AccordingtotheIEEE802.3zstandardrecommendation,atarateof1Gbit/s,a62.5μmcorediametermultimodefibercanonlytransmit270meters;anda50μmcorediametermultimodefibercantransmit550meters.Infact,recentexperimentshaveconfirmedthatusing850nmverticalcavitysurfaceemittinglaser(VCSEL)asthelightsource,atarateof1Gbit/s,astandardmultimodefiberwithacorediameterof50μmcantransmit1750meterswithouterror(5pairsofconnectorsintheline).Anewgenerationofmultimodefiberwithacorediameterof50μmcantransmit2000meterswithouterror(2pairsofconnectorsareincludedintheline).
Anotherreasonforadoptingacorediameterof50μmisthattheadvantagesofmultimodefiberwithacorediameterof62.5μmhavebecomeirrelevantwiththeadvancementoftechnology.Intheearlyandmid-1980s,theoutputpoweroftheLEDlightsourcewaslow,thedivergenceanglewaslarge,andtheconnectorlosswaslarge.Itisnecessarytoconsidertheuseoffiberswithlargecorediameterandnumericalaperturetoinjectasmuchopticalpoweraspossible.Atthattime,nooneseemedtothinkthattherateoftheLANmightexceed100Mbit/s,thatis,thebandwidthperformanceofmultimodefiberwasnotoutstanding,especiallywhenVCSELwasused,opticalpowerinjectionwasnolongeraproblem.Thecorediameterandnumericalaperturearenolongerasimportantasbefore,andthetransmissionrateof10Gbit/shasbecomethemaincontradiction.The50μmcorediametermultimodefiberthatcanprovidehigherbandwidthisfavored.
Lightsource
Inthepast,traditionalmulti-modefiberopticnetworksusedlight-emittingdiodes(LEDs)aslightsources.Thisisaneconomicalandreasonablechoiceinlow-speednetworks.However,thediodeemitslightbyspontaneousemission,andthelaseremitslightbystimulatedemission.Thecarrierlifeoftheformerislongerthanthatofthelatter.Therefore,themodulationrateofthediodeislimitedandcannotbeusedinagigabitratioandabovenetworks.Inaddition,comparedwithalaser,adiodehasalargerbeamdivergenceangleandawiderspectralwidth.Afterinjectingthemultimodefiber,morehigh-ordermodesareexcited,andmorewavelengthcomponentsareintroduced,whichreducesthefiberbandwidth.Fortunately,the850nmverticalcavitysurfaceemittinglaser(VCSEL)notonlyhastheadvantagesoftheabove-mentionedlaser,butthepriceisbasicallythesameasthatoftheLED.TheotheradvantagesofVCSELare:lowthresholdcurrent,itcanbedirectlydrivenbylogicgatecircuitwithoutamplification,andobtainoutputpowerofseveralmilliwattsat2Ggabitrate;itsemissionwavelengthof850nmisnotsuitableforstandardsingle-modefiber,justUsedformultimodefiber.Atthiswavelength,inexpensivesilicondetectorscanbeusedandhavegoodhigh-frequencyresponse;anotherremarkableadvantageisthattheVCSELmanufacturingprocesscaneasilycontrolthedistributionoftheemittedlightpower,whichisusefulforincreasingthebandwidthofmultimodeopticalfibers.Veryadvantageous.Itispreciselybecauseoftheseadvantagesthatthenewgenerationofmultimodefiberstandardswilluse850nmVCSELasthelightsource.
Bandwidth
Accordingtothecomparisonbetweenthelaserandthelight-emittingtubedescribedabove,themultimodefiberusesalaserasthelightsource,anditstransmissionbandwidthshouldbegreatlyimproved.ButpreliminaryexperimentalresultsshowthatsimplyusinglasersinsteadofLEDsaslightsources,thebandwidthofthesystemisnotimprovedbutreduced.AftertheresearchoftheIEEEexpertgroup,itisfoundthatthebandwidthofamultimodefiberisalsorelatedtothemodepowerdistributionorinjectionstateinthefiber.Inthepreformmanufacturingprocess,theaxisoftheopticalfiberispronetorefractiveindexdepression.Inthepast,LEDswereusedasthelightsource,whichwasoverfilled(OFL—OverFilledLaunch).Allmodes(severalhundreds)ofthefiberwereexcited,andeachmodecarrieditsownpartofthepower.Thedistortionoftherefractiveindexofthefibercenteronlyaffectsthedelaycharacteristicsofafewmodes,andtheinfluenceonthefibermodebandwidthisrelativelylimited.ThemeasuredbandwidthofthemultimodefiberiscorrectforthesystemusingLEDasthelightsource.Inotherwords,thebandwidthdatameasuredinthiswaycanbeusedtoestimatethetransmissionrateanddistanceofthesystem.However,whenalaserisusedasalightsource,thelaserspotisonlyafewmicronsandthedivergenceangleissmallerthanthatoftheLED,soonlyafewmodestransmittedinthecenterofthefiberareexcited,andeachmodecarriesaconsiderablepartofthepower,andtherefractiveindexofthefibercenterisdistorted.Theimpactonthedelaycharacteristicsoftheseonlyandfewmodeshassignificantlyreducedthebandwidthofthemultimodefiber.Therefore,thetraditionaloverfillinjection(OFL)methodcannotbeusedtomeasurethebandwidthofamultimodefiberwithalaserasalightsource.
Thenewstandardwillusethelimitedmodeinjectionmethod(RML—RestrictedModeLaunch)tomeasurethebandwidthofanewgenerationofmultimodefiber.Thebandwidthmeasuredbythismethodiscalled"laserbandwidth"or"mode-limitedbandwidth".ThebandwidthmeasuredbyusingLEDasalightsourceiscalled"overfilledinjectionbandwidth".ThetworespectivelyrepresentthebandwidthofthemultimodefiberwhenlaserandLEDareusedforlightsourceinjection.Thestandardsformode-limitedinjectionandbandwidthofmultimodefiberlasersweredraftedbytheTIAFO-2.2.1taskgroup.Thecontentisasfollows:
FOTP-203specifiesthepowerdistributionofthelightsourceusedtomeasurethebandwidthofamultimodefiberlaser.Thelightsourceisrequiredtopassthroughashortmultimodefibercoupling,anditsnear-fieldintensitydistributionshouldsatisfythattheluminousfluxinthecenter30μmrangeisgreaterthan75%,andtheluminousfluxinthecenter9μmrangeisgreaterthan25%.Inthenewstandard,itisnotrecommendedtouseVCSELasalightsourcetomeasurebandwidth.ThisisbecausetheopticalpowerdistributionofVCSELsfromdifferentmanufacturersisverydifferent.
FOTP-204specifiestheuseofmode-limitedfibertocouplethelightsourceintoamultimodefiberforlaserbandwidthmeasurement.Themode-limitedfiberisusedtofiltertheoverfilledstateandlimittheexcitationofthehigh-ordermodeofthemultimodefiber.Themode-limitedfiberisasectionofgradedindexmultimodefiberwithacorediameterof23.5μmandanumericalapertureof0.208.Therefractiveindexgradientindexofthismultimodefiberiscloseto2.Under850nmand1300nmoverfillconditions,thereshouldbeabandwidthgreaterthan700MHz.km.Thelengthofthemode-limitedfibershouldbegreaterthan1.5meterstoeliminatetheleakagemode,andlessthan5meterstoavoidtransientloss.Thecorediameterof23.5μmischosenbecausetheinjectionstateitproducesisclosesttotheVCSEL.
Lightsourceinjection
Introduction
Inactualuse,thecouplingoflaserandmultimodefibercanfollowtherecommendedmethodofGbit/sEthernetstandard:
Biasinjection
Inordertoavoidthebandwidthdegradationcausedbythedirectinjectionofthelaserintothemultimodefiber,thestandardstipulatesthatthemodeadjustmentconnection(ModeConditioningPatchCord—MCP)isusedtocouplethelaseroutputintothemultimodefiber.Modefiber.Themodeadjustmentconnectionisashortsingle-modefiber,oneendofwhichiscoupledwiththelaser,andtheotherendiscoupledwiththemultimodefiber.Thestandardstipulatesthattheoutputspotofthesingle-modefiberdeliberatelydeviatesfromtheaxisofthemulti-modefiberbyacertaindistance,andtheallowabledeviationrangeis17~24μm.Thepurposeistoavoidthecentralrefractiveindexdepression,butnotdeviatetoofar,butselectivelyexciteasmallgroupLowerordermode.
Centerinjection
Fortheidealrefractiveindexprofile,themultimodefiberwithoutacenterrecesscanusecenterinjectioninsteadofmodeadjustmentconnections.Theadvantageofthisisthatitcaneffectivelyincreasethelaserbandwidthofthemultimodefiber,reducethecomplexityofthenetworksystemandreducethesystemcost,andamodeadjustmentconnectionisabout80-100USdollars.
Thedifferencebetweensingle-modeandmulti-mode
1,single-modetransmissiondistanceislong
2,multi-modetransmissionbandwidthislarge
3,single-modeNochromaticdispersion,reliablequality
4.Single-modeusuallyuseslaserasalightsource,whichisexpensive,whilemulti-modeusuallyusescheapLED
5.Single-modeismoreexpensive
p>
6.Multi-modeischeap,andshort-distancetransmissionispossible